Processes of chemical nickel plating and baths therefor



G. GUTZEIT Nov. 9, 1954 v PROCESSES OF CHEMICAL NICKEL PLATING AND BATHSTHEREFOR Filed April 23, 1952 In Ill? F Wm MM 0 M 55 1- D? m m m m l 9 75 9 s, s k

Ini fial pH v INVENTOR Gregoire Garza/7 Alb s.

. down rather rapidly United States Patent C) PROCESSES F CHEMICALNICKEL PLATING' AND BATHS THEREFOR Gregoire Gutzeit, Highland, Ind.,assignor to General American Transportation Corporation, Chicago, Ill-.,a corporation of New York Application April' 23, 1952, Serial No.283,825

13 Claims. (Cl. 117-130),

The present invention relates to improved processes of chemical nickelplating of catalytic materials employing baths of the nickelcation-hypophosphite anion type, and to improved baths employed in suchprocesses.

The chemical nickel plating of a catalytic material employing an aqueousacid bath of the nickel cationhypophosphite anion type is based upon thecatalytic reduction of nickel cations. to metallic nickel and thecorresponding oxidation of hypophosphite anions to phosphite anions withthe evolution of hydrogen gas at the catalytic surface. The reactionstake place whenv the body of catalytic material is immersed in the bath,and the exterior surface 'of the body of catalytic material iscoatedwith nickel. The following elements are examples of catalyticmaterials which may be nickelplated: copper, silver, gold, beryllium,boron, germanium, aluminum, thallium, silicon, carbon, vanadium,molybdenum, tungsten, chromium, selenium, tellurium, titanium, iron,cobalt, nickel, palladium and platinum; and the following elements areexamples of non-catalytic materials which ordinarily may not benickel-plated": bis-- muth, cadmium, tin, lead and manganese. Theactivity of these catalytic materials varies considerably; and thefollowing elements are particularly good catalystsv in the chemicalnickel plating bath: aluminum, copper, chromium, cobalt, iron, nickeland palladium. The chemical nickel plating process is autocatalyticsince both the original surface of the body being plated and the nickelmetal that is deposited on the surface thereof are bothcatalytic; andthe reduction of the nickel cations to metallic nickel in the bathproceeds until all of the nickel cations have been reduced to metallicnickel, in the presence of an excess of hypophosphite anions, or untilall of the hypophosphite anions havebeen oxidized to phosphite anions,in the presence of an excess of nickel cations. Actually the reactionsare slowed-- as time proceeds because the anions, as contrasted with thecations, of the nickel salt. that. is dissolved in the bath combine withthe hydrogen cations to form an acid, which, in turn, lowers the pH ofthe bath, tending to dissolve the nickel deposit. Also the reducingpower of the hypophosphite anions is decreased as the pH value of thebath decreases. Moreover, there is a tendency for the bath, as the pHvalue thereof is decreased, to become unstable with the'formation of ablack precipitate that comprises a random chemical non-catalyticreduction of the nickel cations; which formation of the blackprecipitate is also favored by a high absolutev concentration ofhypophosphite anions in the bath. Of course, this formation of blackprecipitate comprises a. decomposition of the bath; and is, furtherobjectionable in. that it causes the nickel deposit to be coarse andrough.

For the dual purposes of increasing the stability of the bath(preventing the formation of the black precipitate mentioned), and ofincreasing the normal plat ing rate of the bath, various baths of thepresent type have been suggested employing different additives thatserve eitheras buffers or as exaltants. For example, in the copendingapplication of Gregoire Gutzeit and- Abraham Krieg, Serial No. 194,656,filed November 8', 1950, now Patent No. 2,658,841, granted November 10,,1953, there is disclosed a chemical nickel plating bath of the nickelcation-hypophosphite anion type that contains as an additive a butter inthe form of a salt of an organic acid, and specifically sodium acetate;and in the copending application of Gregoire Gutzeit and Ernest J.

Ramirez, Serial No. 204,424, filed January 4, 1951, now Patent No.2,658,842, granted November 10, 1953, there is disclosed a chemicalnickel plating bath of the nickel cation-hypophosphite anion type thatcontains as anadditive an exaltantin the form of a simple short chainsaturated aliphatic dicarboxylic acid, and specifically sodiumsuccinate.

The present invention involves the discovery that the plating'rate of abath of the nickel cation-hypophosphite anion type may be greatlyincreased without rendering the bath unstable by employing as anadditive a small amount of a soluble fluoride; which additive may beemployed: either in. lieu of, or in conjunction with, an additive of thecharacter of that employed in the baths mentioned above. It ispostulated that the fluoride anions form heteropoly-acid anions with thehypophosphite anions and that the reducing power of these complex anionsis much greater than that of the simple hypophosphite anions or that ofthe complex anions produced by the organic acids mentioned. Thesecomplex anions seem to be fluoro-hypophosphite anions; and in any case,the phenomenon of exaltation is achieved; whereby the increase in therate of catalytic oxidation of the hypophosphite anions to phosphiteanions can be measured experimentally by determining the amount ofhydrogen gas evolved per unit of time, with and. Without the addition ofthe fluoride anions, with, all other conditions in the bath remainingthe same.

It is, therefore, the primary object of the present invention to providean improved catalytic nickelplating process of the character described,in which the reaction involved is carried out more efiiciently, rapidlyand perfectly than heretofore, thereby rendering the process moredesirable from a commercial standpoint.

A further object of the invention is to provide an improved process ofthe character described that employs a bath of the nickelcation-hypophosphite anion type containing as an additive fluorideanions.

Another object of the invention is to provide an improved aqueous acidbath of the nickel cation-hypophosphite anion type containing fluorideanions.

A further object of the invention is to provide an improved process ofchemical nickel plating employing a bath of the nickelcation-hypophosphite anion type, which includes as exaltants, both asoluble salt of a simple short chain saturated aliphatic dicarboxylicacid and a soluble fluoride.

These and other objects and advantages of the present invention will beunderstood, from the foregoing and the following description, taken withthe accompanying drawmg, n wh1ch the single figure shows the variationsin the Weight of the nickel plating deposited upon standard test sampleswith changes in the initial pH of the bath and. with dilferent absoluteconcentrations of the fluoride anions therein.

In accordance with the process of the present invent1on, the:art 1cle tobe nickel plated, and normally formed of-a catalytic material, isproperly prepared by mechanically cleamng, degreasing and lightpickling, according to the standard practice in electroplatingprocesses. For example, in thenickel plating of a steel object, it iscustomary mechanically to clean the rust and mill scale from the ob ect,to degrease the object, and then lightly to pickle the object in asuitable acid, such as hydrochloric acid. I The article is then immersedin a suitable I of nickel cations, hypophosphite anions and fluorideanions, the pH of the bath having been, if necessary, adusted to anoptimum value by the addition of a suitable agent, and the. bath havingbeen heated to a temperature ust below its, boiling point, such as 997C., at atmosphenc pressure. Almost immediately, hydrogen bubbles areformed on the catalytic surface of the steel object and escape in asteady stream from the bath, while the surface of the steel object isslowly coated with metallic nickel (containing some phosphorus). Thereaction is continued until the color of the bath (green at the start)shows the absence of nickel, or until the evolution of hydrogen stops,or until it'is determined that the re.-

upon theisteel object. Of course, the steel object is then Patented Nov.9,, 1.95.4

3 removed from the bath and rinsed ofl with water, and is then ready foruse. In general, best chemical nickel plating results are obtained whenthe ratio (V/A) between the volume of the bath (cm. and the surface areaof the object being plated (cm?) is below 10.

With respect to the composition of the bath, it essentially comprises anaqueous acid solution containing nickel cations, hypophosphite anions,and fluoride anions, and may be formed by dissolving in a suitableacidwater solution, a soluble nickel salt, a soluble hypophosphite, anda soluble fluoride. For example, the nickel cations may be derived fromnickel chloride (commercial grade); the hypophosphite anions may bederived from sodium, potassium, lithium, calcium, magnesium, strontium,barium, etc., hypophosphites, or various combinations thereof; and thefluoride anions may be derived from sodium, potassium, etc., fluorides.In passing, it is noted that certain alkaline cations that may be thusintroduced in the bath appear to retard 1n the process the rate ofnickel deposition with respect to other cations; for example, bariumcations appear to retard the rate of nickel deposition with respect tosodium and potassium cations. Specifically, a suitable bath may beformed in an exceedingly simple manner by dissolving in a hydrochloricacid-water solution commercial nickel chloride, sodium hypophosphite andsodium fluoride. The desired pH of the bath is established by theintroduction thereinto of additional hydrochloric acid, and isappropriately adjusted by the addition thereto of a weak alkali,preferably sodium bicarbonate. Also, it will be understood that thefluoride anions and some of the hydrogen cations may be derived byintroducing hydrofluoric acid, instead of sodium fluoride, into thebath; although ordinarily hydrofluoric acid is more troublesome tohandle than hydrochloric acid.

The terms cation, anion and ion as employed herein include the totalquantity of the corresponding elements that are present in the bath; i.e., both undissociated and dissociated material. In other words, 100%dissociation is assumed when the terms noted are used in connection withmolar ratios and concentrations in the bath.

For the purpose of demonstrating the exalting effect, with reference tothe increase in the plating rate, of the fluoride anion additive, aseries of comparative plating tests were conducted. In the first ofthese plating tests a steel sample was plated for 10 minutes in a simpleaqueous acid bath of the nickel cation-hypophosphite anion typemaintained at a temperature of 98 C. The bath employed in this firstplating test comprised no buffer or exaltant and consisted essentiallyof nickel cations (0.09 mole/ liter) derived from commercial nickelchloride, hypophosphite anions (0.225 mole/liter) derived from sodiumhypophosphite, and enough Water to produce a liter of solution, theinitial pH of the bath being adjusted with hydrochloric acid toapproximately 4.5. In this first plating test, the plating rate,expressed in gm./cm. /min., was 0.585X10- A second plating test wasconducted under conditions identical to the first plating test, exceptthat the bath was modified by the addition of fluoride anions (0.10mole/liter) derived from sodium fluoride. In this second plating test,the plating rate, expressed in gm./cm. /min., was 3.58X10- Accordingly,in the second plating test, the plating rate Was increased, with respectto that of the first plating test, by over 500%; which circumstanceclearly demonstrates the exalting eifect of the fluoride anions; andmoreover, the first and second plating tests clearly demonstrate thatthe exalting effect of the fluoride anions is not a result ofinteraction with organic additives (buflers and other exaltants), sincethe bath employed in the second plating test contained no organicadditives.

A third plating test was conducted employing a steel sample for a timeinterval of 120 minutes and utilizing a bath identical to that of thesecond plating test, except that a small amount of an organic exaltant,sodium succinate (0.02 mole/liter) was added thereto. The plating rateobtained in the third plating test, expressed in gm./cm. /min., was5.35Xl which represents an increase of about 815% in the plating rateover that obtained in the first plating test, and a considerableincrease in the plating rate over that obtained in the second platingtest. Thus a comparison of the second and third plating testsdemonstrates that the combination of the 4 fluoride anion and thesuccinate anion additives are productive of a composite exalting effect.

A fourth plating test was conducted upon a steel sample for ten minutesemploying a bath maintained at a temperature of 98 C. This bath wassubstantially identical to that of the first plating test, except thatit was modified by employing as an additive a buffer in the form ofsodium citrate and a slightly different pH. More particularly, the bathincluded citrate anions (0.03 mole/liter) derived from sodium citrate;and the initial pH of the bath was 4.6 obtained by hydrochloric acid. Inthis fourth plating test, the plating rate, expressed in gms./cm. /min.,was 1.32 10 A fifth plating test identical to the fourth plating testwas conducted, except that the bath was modified by the addition offluoride anions (0.10 mole/liter) derived from sodium fluoride. In thisfifth plating test, the plating rate, expressed in gms./cm. /min., was3.l8 10 A comparison of the fifth plating test with the fourth platingtest demonstrates that the exalting effect of the fluoride anionadditive to the citrate bath was productive of an increase of in theplating rate.

In order to demonstrate the increased plating rates that are obtained byemploying the fluoride anion additive in a nickel cation-hypophosphiteanion bath of the character of that disclosed in the Gutzeit and Kriegapplication, previously mentioned, two series of comparative platingtests were conducted, first without and then with the additivementioned, and with variable initial pH. In each of these plating tests,a steel sample having an area of 20 cm. was plated for 60 minutes in 50cc. of the bath at a temperature of 98 C. In the first series of theseplating tests, the bath contained nickel cations (0.0704 mole/liter)derived from commercial nickel chloride; hypophosphite anions (0.225mole/liter) derived from sodium hypophosphite; and acetate anions (0.120mole/liter) derived from sodium acetate. In four of these plating testsof this first series,

.the initial pH were respectively: 5.01, 5.38, 5.79 and 6.33; and therespective weight gains, expressed in gm., were: 0.1832, 0.1952, 0.2064and 0.2063. In the second series of these plating tests, the principalcomposition of the baths was identical to that of the baths employed inthe first series of plating tests, except that each of the bathscontained the fluoride anion additive (0.10 mole/liter) derived fromsodium fluoride. In four of these plating tests of this second series,the initial pH were respectively: 4.53, 4.93, 5.34 and 6.35; and therespective weight gains, expressed in gm., were: 0.2262, 0.2486,

0.2645 and 0.2544. Accordingly, these two series of plating testsdemonstrate that the combination of the fluoride anion additive and theacetate anion buffer are productive of a composite exaltation of theplating rate in a bath of the nickel cation-hypophosphite anion type.For example, in a bath of the character noted, at a pH of about 5.35,the weight gains of test samples were 0.1952, gm. of nickel without thefluoride anion addition, and 0.2685 gm. of nickel with the fluorideanion addition in the amount of only 0.10 mole/liter of fluoride anion,representing roughly 35% improvement in the nickel plating rate.

In order to demonstrate the increased plating rates that are obtained byemploying the fluoride anion additive in a bath of the general characterof that disclosed in the Gutzeit and Ramirez application previouslymentioned, a further series of plating tests No. 1 to No. 13, inclusive,were conducted, employing certain variables in the compositions of thebaths, as explained more fully hereinafter, and as set forth in thetable appearing subsequently. First, it is noted that in plating testNo. 1, the bath contained as an exaltant only the anions of the simpleshort chain saturated aliphatic dicarboxylic acid mentioned; whereas inplating tests Nos. 2 to 13, inclusive, the corresponding baths containedas exaltants not only the anions of the simple short chain saturateddicarboxylic acid mentioned but also fluoride anions. In each of platingtests Nos. 1 to 13, inclusive, 50 cc. of the plating bath was employed,and the nickel plating was deposited upon a steel sample having an areaof 20 cm. during a test time interval of 60 or 66 minutes, as indicatedin-the table mentioned. Also, in

each test, the V/A ratio between the volume of the L plating bath in cm.and the surface area of the steel sample in cm. was 2.5. Further, ineach test, the bath comprised 0.09 mole/liter of nickel cations derivedassigns:

from commercial. nickel chloride, 0.225 mole/liter of hypophosphiteanions derived from sodium hypophosphite, and 0.06 mole/liter ofsuccinate anions derived from sodium succinate. In the various baths,the

principal variants were the contents of fluoride anionsderived fromsodium fluoride and the initial pH values thereof, as illustrated in thetable:

tant in. the form of the succinate anions isnot use'd cinic. acid, andlikewise the exaltant in the form: of the fluoride anion is not used upin the plating operation,

Test No l 2 3 4 5 0" 7' 8 9 10 11 12 13 Mole/liter F- None 0. 10 0.100.10 0.10 0. 10 0. 15 g 0.15 0.15 0.15 0.15 0. 225 0. 405. Initial pH 4.60- 4. 06 4. 50 l 5.00 5. 50' 5.94 5.01 5. 32' 5:68 6.18 6. 82 4. 60 4.60 Duration of test (min 60 60 60v 60 60 60 66 66 66 66 66 60 68 Weightgain (g'ms.) 0 0975 0.1387 0.2297 0. 2495 0.2599 0.2643 0.2021 0. 26250. 2374 0. 2445 0.2354 0.1644: 0. 1480 a p pp c (l) 2) i (0 Time-toblack prec. (mins.) i 26 27 45 45 34'. 34

1 Br! ht and smooth. 2 Slig tly rough. Semi-bright.

In the drawing, the results of test No. 1 are plotted as the point A;the results of plating tests Nos. 2 to 6, inclusive, are plotted as thecurve B; the results of plating tests Nos. 7 to 11, inclusive, areplotted as the curve C; and the results of plating tests Nos. 12 and. 13are respectively plotted as the points D and E. In each case, the Weightgain-in grams. of the standard test sample is plotted as the ordinate,and the initial pH of the bath is plotted as the abscissa. A comparisonof the curve B with the point A dramatically illustrates that theaddition of the exaltant comprising the fluoride anions to the otherwiseidentical bath produces the very remarkable increases in nickel platingrates. For instance, a comparison of plating tests Nos. 1 and 6 revealsthat the nickel plating rate of the bath of plating test No. 6 comprisesa 271% increase over the nickel plating rate of the bath of plating testNo. 1, due substantially entirely to the addition of the fluoride anionsin the small amount of 0.10 mole/ liter in conjunction with the morefavorable initial pH value of the bath. Also the curve B illustratesthat the optimum value of the pH of the bath falls within the relativelynarrow range 5.5 to 6.0. In each of plating tests Nos. 1 to 7.inclusive. it is noted that the nickel coating deposited upon the testsample was bright and smooth in appearance, and was otherwise highlysatisfactory, as indicated by the table.

From the curve C, it will be observed that while increased nickelplating rates were obtained in plating tests Nos. 7 to 11, inclusive,with respect to the point A, corresponding to plating test No. 1, whenthe corresponding baths contained as an exaltant 0.15 mole/liter offluoride anions, the corresponding nickel deposits were slightly rough,or at least only semi-bright; whereby the nickel deposits were notconsidered to be altogether satisfactory. In passing, it is noted whenthe nickel deposit upon the test sample is even slightly rough, thecorresponding measurement of the weight of the deposit is considered tobe unfavorably high by virtue of the small amounts of trapped blackprecipitate. Accordingly, the curve C indicates that there is a definiteupper limit to the maximum amount of exaltant in the form of fluorideanions that may be advantageously introduced into the plating bath.Specifically, when the plating bath contains the exaltant in the form ofthe fluoride anions in an amount somewhat in excess of 0.12 mole/liter,the stability of the plating bath is adversely affected, as indicated bythe early formation of the black precipitate and the consequentproduction of slightly rough nickel deposits upon the test samples, asclearly indicated by plating tests Nos. 7 to 11, inclusive. Finally,plating tests Nos. 12 and 13, the results of which are respectivelyplotted at the points D and E, demonstrate that when the amount ofexaltant in the form of the fluoride anions in the baths issubstantially increased above the content noted above, bright and smoothnickel deposits may still be obtained employing lower pH values of thebaths, but that in these cases, the increase in the nickel plating rateis relatively small with respect to test No. l, plotted as the point A.Accordingly, it may be stated that an excess of the exaltant in the formof the fluoride anions in the bath above the content of about 0.12mole/liter is detrimental as the excess either impairs the stability ofthe bath, as indicated by plating tests Nos. 7 to 11, incluan exhaustedplating bath can be regenerated by readjusting the nickel cation contentand the hypophosphiteanion content and by neutralizing the acidsto theproper pH value by means of a weak alkali, such as sodium: bicarbonate.Accordingly, this technique has the advantage that a brighter nickelcoating at a greatly increased plating rate may be produced, whereby thetime-interval required to produce a nickel coating of a predeterminedthickness upon a catalytic object is greatly minimized.

In view of the foregoing, it is apparent that there has been provided animproved process of plating a catalytic material with nickel and animproved bath of the nickel cation-hypophosphite anion type for use inthe process, wherein the plating rate of the process is greatlyincreased with respect to prior processes of this character, by virtueof the incorporation in the bath of the exaltant in the form of a smallquantity of soluble fluoride.

While there has been described what is at present con sidered to be thepreferred embodiment of the invention,

- it will be understood that various modifications may be made therein,and it is intended to cover in the appended claims all suchmodifications as fall within the true spirit and scope of the invention.

What is claimed is:

l. The process of chemically plating With nickel a solid bodyessentially comprising an element selected from the group consisting ofcopper, silver, gold, aluminum, iron, cobalt, nickel, palladium andplatinum, which comprises contacting said body with a bath consistingessentially of an aqueous solution of a nickel salt and a hypophosphiteand a salt of a simple short chain saturated aliphatic dicarboxylic acidand a fluoride, wherein the absolute concentration of fluoride ions insaid bath expressed in mole/liter is between 0.01 and 0.12.

The process set forth in claim 1, wherein the ratio between nickel ionsand hypophosphite ions in said bath expressed in molar concentrations isbetween 0.25 and 1.60, wherein the absolute concentration of hophosphite ions in said bath expressed in mole/liter is etween 0.15 and1.20, and wherein the absolute concentration of dicarboxylic ions insaid bath is at least two carboxyl groups for every nickel ion that canbe deposited. 1

3. The process set forth in claim 2, wherein the initial pH of said bathis in the approximate range 4.5 to 6.5.

4. The process of chemically plating with nickel a solid bodyessentially comprising an element selected from the group consisting ofcopper, silver, gold, aluminum, iron, cobalt, nickel, palladium andplatinum, which comprises contacting said body with a bath consistingessentially of an aqueous solution of anickel salt and a hypophosphiteand a succinate and a fluoride, wherein the absolute concentration offluoride ions in said bath expressed in mole/ liter is between 0.01 and0.12.

5. The .process set forth in claim 4, wherein the ratio between nickelions and hypophosphite ions in said bath expressed in molarconcentrations is between 0.25 and 1.60, wherein the absoluteconcentration of hypophosphite ions in said bath expressed in mole-literis between 0.15 and 1.20, and wherein the absolute concentration ofsuccingte ions in said bath expressed in mole/liter is at least 0.0

6. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution of a nickel salt and ahypophosphite and a salt of a simple short chain saturated dicarboxylicacid and a fluoride, wherein the absolute concentration of fluoride ionsin said bath expressed in mole/ liter is between 0.01 and 0.12.

7. The bath set forth in claim 6, wherein the ratio between nickel ionsand hypophosphite ions in said bath expressed in molar concentrations isbetween 0.25 and 1.60, wherein the absolute concentration ofhypophosphite ions in said bath expressed in mole/ liter is between 0.15and 1.20, and wherein the absolute concentration of dicarboxylic ions insaid bath is at least two carboxyl groups for every nickel ion that canbe deposited.

8. The bath set forth in claim 7, wherein the initial pH of said bath isin the approximate range 4.5 to 6.5.

9. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous acid solution of nickel chloride,sodium hypophosphite, sodium succinate and sodium fluoride, wherein theabsolute concentration of fluoride ions in said bath expressed in mole/liter is between 0.01 and 0.12, and wherein the initial pH of said bathis in the approximate range 4.5 to 6.5.

10. The process of chemically plating with nickel a solid bodyessentially comprising an element selected from the group consisting ofcopper, silver, gold, aluminum, iron, cobalt, nickel, palladium andplatinum, which comprises contacting said body with a bath consistingessentially of an aqueous solution of a nickel salt and a hypophosphiteand a fluoride, wherein the absolute concentra- 8 tion of fluoride ionsin said bath expressed in mole/liter is between 0.01 and 0.12.

11. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution'of a nickel salt and ahypophosphite and a fluoride,

the absolute concentration of fluoride ions in said bath expressed inmole/ liter being between 0.01 and 0.12.

12 A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution of a nickel salt and ahypophosphite and a buffer in the form of a salt of an organic acid andan exaltant in the form of a fluoride, wherein the absoluteconcentration of fluoride ions in said bath expressed in mole/liter isbetween 0.01 and 0.12.

13. A bath for the chemical plating of a catalytic material with nickelconsisting essentially of an aqueous solution of a nickel salt and ahypophosphite and an exaltant, said exaltant consisting essentially of asalt of an organic acid and a fluoride, wherein the absoluteconcentration of fluoride ions in said bath expressed in mole/liter isbetween 0.01 and 0.12.

References Cited in the file of this patent UNITED STATES PATENTS NumberName Date 2,532,283 Brenner Dec. 5, 1950 2,580,773 Heiman Jan. 1, 1952

1. THE PROCESS OF CHEMICALLY PLATING WITH NICKEL A SOLID BODYESSENTIALLY COMPRISING AN ELEMENT SELECTED FROM THE GROUP CONSISTING OFCOPPER, SILVER, GOLD, ALUMINUM, IRON, COBALT, NICKEL, PALLADIUM ANDPLATINUM, WHICH COMPRISES CONTACTING SAID BODY WITH A BATH CONSISTINGESSENTIALLY OF AN AQUEOUS SOLUTION OF A NICKEL SALT AND A HYPOPHOSPHITEAND A SALT OF A SIMPLE SHORT CHAIN SATURATED ALIPHATIC DICARBOXYLIC ACIDAND A FLUORIDE, WHEREIN THE ABSOLUTE CONCENTRATION OF FLUORIDE IONS INSAID BATH EXPRESSED IN MOLE/LITER IS BETWEEN 0.01 AND 0.12